Inorganic pigments containing cadmium, lead and chromium-(VI) have in recent years increasingly been regarded as harmful on ecological and toxicological grounds. In a search for alternative products, systems containing bismuth oxide in particular have been tested for their coloristic properties.
Among these, BiVO.sub.4, which also occurs as a mineral and the technical production of which is described in DE-C 422 947, has been in the forefront of these investigations. The method described there, precipitation of bismuth nitrate with alkali metal vanadate solution followed by calcination of the precipitated gel, has been further developed in DE-A 2 727 864 to provide a process for the preparation of a luminous, primrose coloured bismuth vanadate with pigment properties. Multiphase pigments containing additives such as silicates, phosphates or sulphates have also been developed with a view to reducing the cost of bismuth vanadate which is in itself expensive. These pigments are described in German Offenlegungs-schriften DE-A 2 933 778 (BiVO.sub.4 -BiPO.sub.4 -AlPO.sub.4), DE-A2 940 185 (BiVO.sub.4 - BiPO.sub.4 - MeO) and DE-A 3 004 083 (BiVO.sub.4 - BaSO.sub.4).
Modifications of the BiVO.sub.4 structure by the incorporation of foreign ions into the lattice have also been carried out for improving the technical pigment properties. Bismuth vanadate pigments containing molybdenum and tungsten belong to the state of the art in this field (DE-A 3 106 625, DE-A 3 135 281, DE-A 3 221 338, EP-A 74 049 and EP-A 239 526). The products disclosed contain mixtures of structurally modified BiVO.sub.4 and Bi-molybdates and alkaline earth metal tungstates.
In order to assess the new class of substances claimed in the present invention by comparison with the structures known in the art, it is necessary to distinguish between four different main modifications of BiVO.sub.4 : BiVO.sub.4 occurring naturally as the mineral pucherite has an orthorhombic structure - space group Pnca with a.sub.o =5.33.ANG., b.sub.o =5.05.ANG. and c.sub.o =12,00.ANG. (J. Granzin and D. Pohl, Z. f. Kristallografie, 169, 289-294 (1984) and thus differs substantially from the basic synthetic type which crystallises in a monoclinically distorted variation of the CaWO.sub.4 lattice (Scheelite type) with the space group I 2/a (W. I. F. David and A. M Glazer, Phase Transitions, Vol. 1 (1979) 155-170). For increasing the stability of the synthetic bismuth vanadate lattice, however, it is the pure tetragonal modification of space group I 4.sub.1 /a which is required. This tetragonal BiVO.sub.4 with Scheelite structure is obtained in the presence of the above mentioned foreign ions such as Mo or W, alone or in combination with alkaline earth metal ions. According to H. Wienand and W. Ostertag (The Bulletin of the Bismuth Institute, 53 (1988) 1-4), BiVO.sub.4 prepared in the presence of sodium molybdate has a tetragonal structure with the lattice constants a.sub.o =5.147.ANG. and c.sub.o =11.722.ANG..
Zhang Wenpu et al (The Bulletin of the Bismuth Institute, 56 (1988) 11) proved that the monoclinic phase changes into the required tetragonal phase if Mo or W is added in the process of synthesis. The particulars given in Patent Specification EP-B 239 526 also indicate that the Mo- and W-containing types of BiVO.sub.4 claimed there have the tetragonal Scheelite type crystal structure.
There is another tetragonal BiVO.sub.4 modification which must be distinguished from the Scheelite type. This modification crystallises in the zirconium type (ZrSiO.sub.4 structure of space group I 4.sub.1 /amd) but is unsuitable as pigment on account of its much paler yellow.
None of the above-mentioned synthetic BiVO.sub.4 structures, however, have sufficient resistance to alkaline attack or exposure to heat for technical use as pigments; these resistances are basic requirements, for example for the incorporation of the pigment in plastics. Moreover, all these pigments are more or less sensitive to light, with the disadvantageous result that they are subject to greying, e.g. when used in lacquers.
It is an object of the present invention to provide bismuth vanadate pigments which are free from the above-described disadvantages.